Digital computers are being used today to perform a wide variety of tasks. Many different areas of business, industry, government, education, entertainment, and most recently, the home, are tapping into the enormous and rapidly growing list of applications developed for today's increasingly powerful computer devices.
As computer systems become increasingly ubiquitous and widespread, there is increasing interest in improving the performance and software execution speed of the computer systems. One of the methods used by designers to increase software execution speed is to increase the processor “clock speed.” Clock speed refers to the rate at which digital systems (graphic processor units, central processor units, digital signal processors, etc.) step through the individual software instructions. For example, with many microprocessor designs, one or more instructions are executed per clock cycle. Increasing the number of clock cycles per second directly increases the number of instructions executed per second.
Another method used by designers is to increase the density of the electrical components within integrated circuit dies. For example, many high-performance integrated circuit processors include tens of millions of transistors integrated into a single die (e.g., 60 million transistors or more). As density increases, the clock speeds possible within a given design also increase, for example, as circuit traces are packed ever more closely together.
Another method for increasing performance is to increase the efficiency of heat removal from a high-density, high-performance integrated circuit. As component density increases and clock speed increases, the thermal energy that must be dissipated per unit area of silicon also increases. To maintain high-performance, stable operating temperature must maintained. Accordingly, the use of carefully designed heat dissipation devices (e.g., heat sink fans, liquid cooling, heat spreaders, etc.) with high-performance processors has become relatively standardized.
Performance enhancing techniques, such as increased component density, increased clock speed, and increased heat dissipation, are carefully balanced in order to obtain an optimum performance level. Processor circuit density, processor clock frequency, and thermal dissipation are balanced to provide high-performance while maintaining some margin of safety. For example, if a processor is clocked at too high a frequency, excessive power consumption occurs, leading to overheating of the components of the processor. Over heating leads to computational errors, unpredictable behavior, or even physical destruction of processor. As more and more functions are integrated into ever more densely packed semiconductor dies, the clock speed can be increased, however, the resulting increased switching activity leads to greater heat generation. These factors are balanced to provide an optimal performance for given device.
There exists particular problems, however, with respect to thermal transients and changing thermal conditions. As described above, integrated circuit performance is dependent upon the temperature of the underlying silicon comprising the integrated circuit (e.g., electron mobility, etc.). Generally, for given semiconductor integrated circuit, cooler semiconductor temperatures yields faster performance (e.g., higher electron mobility) than warmer semiconductor temperatures. This effect is often evidenced in the “ramp rate” of the rising edges of various signals within a processor. For example, the rising edges of the clock signal of a cool (e.g., 20C) processor ramp more steeply with respect to time than the rising edges of a hot (e.g., 100C) processor. Performance factors (e.g., clock frequency, component density, thermal dissipation) are typically optimized with respect to expected steady-state operating conditions. This leads to problems when changing temperature conditions are encountered.
One example of a thermal transient is a case where a processor (e.g., of a laptop computer system) is powered up from an “off” state. When a processor is powered-up, power is applied to cold silicon, leading to exceptionally fast performance. In this initially powered state, the excessive ramp rates of the signals can lead to jitter on the rising edges of the signals. The excessive electron mobility and the resulting jitter can cause an unacceptable amount of crosstalk between the circuit traces. This effect is even more problematic in more modern processor dies, having very densely packed, very highly integrated logic components. As component density increases, circuit traces become packed more closely together, and thus become more susceptible to crosstalk. Exceptional amounts of jitter and noise can cause excessive crosstalk.
As described above, design engineers carefully balance increased component density, increased clock speed, and increased heat dissipation to obtain maximum performance. The crosstalk, jitter, and noise caused by changing temperature thus becomes an important performance limiting factor. With prior art schemes, an engineer must design in sufficient performance margin to account for the crosstalk and jitter caused by thermal transients (e.g., startup, etc.). This leads to less than the full performance potential of a given device being obtained.